In the face of the existing HIV-1 epidemic, an effective mix of prevention modalities are needed to begin to reduce the incidence of HIV-1. This includes the used of topical microbicides. These are products that would be used to prevent the sexual transmission of HIV-1;including receptive anal intercourse. Project 1, Nonclinical Strategies for Refining Combination Rectal Formulations, of this U19 will strive to ensure that a safe and effective rectal microbicide will be developed. To date, the Phase 2b trials of "first generation" microbicide products, nonoxynol-9, SAVVY, cellulose sulfate, and Carraguard, have either failed to show effectiveness or showed harm and were halted early. This has lead the way to using "second generation" microbicide products that encompass the antiretroviral (ARV) drugs, UC781 and tenofovir. We have developed an innovative testing algorithm to evaluate safety and effectiveness of ARV rectal microbicide (RM) formulations that will be developed by this U19 (Core C). This includes the use of a polarized colorectal explant culture system which to date has generated results that has been reflective of nonhuman primate and human Phase 1 clinical safety data as well as predictive of nonhuman primate and human Phase 2b clinical effectiveness data. It is of interest to determine if these RM formulations are effective indepentent of coitus as well as in the presence of semen. Finally, in anticipation of developing combinations of microbicides that will have multiple mechanisms of action to improve preventative effectiveness, molecules that block or prevent the fusion of HIV-1 with the target cell will be tested for antagonism or synergy with the ARV drugs. The ultimate goal of this work will be to ensure that what is developed in this U19 for RM will be safe and have the potential to be highly effective when tested in a clinical trial.